High strength galvanized steel sheet and method for producing the same

ABSTRACT

A high-strength hot-dip galvanized steel sheet that even on the premise of ordinary CGL heat cycle, has a low yield stress and excels in resistance to natural aging and baking hardenability without reliance on the use of expensive Mo; and a process for producing the same. The constituent composition thereof comprises 0.01 to less than 0.08% C, 0.2% or less Si, more than 1.0 to 1.8% Mn, 0.10% or less P, 0.03% or less S, 0.1% or less Al, 0.008% or less N and more than 0.5% Cr so that the relationship 1.95≦Mn(mass %)+1.3Cr(mass %)≦2.8 is satisfied and comprising the balance iron and unavoidable impurities. The structure thereof has a ferrite phase and a martensite phase of 2 to 15% area ratio, and the cumulative area ratio of pearlite phase and/or bainite phase is 1.0% or less. In the production of this hot-dip galvanized steel sheet, the temperature and cooling rate are controlled during the annealing/plating operation subsequent to cold rolling.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/062878, withan international filing date of Jul. 10, 2008 (WO 2009/008553 A1,published Jan. 15, 2009), which is based on Japanese Patent ApplicationNos. 2007-181946, filed Jul. 11, 2007, and 2008-177466, filed Jul. 8,2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a galvanized steel sheet which is suitablyused, for example, in an automobile and a home electric appliance fieldand which has a low yield stress and superior anti-aging property andbake hardenability and to a method for producing the galvanized steelsheet.

BACKGROUND

In recent years, reduction in thickness of a steel sheet to improve fuelconsumption by lightening an automobile body and increase in strength ofa steel sheet to improve safety have been pursued. However, the increasein strength of a steel sheet generally causes deterioration ofpress-formability, i.e., wrinkles, which are called surface distortions,on the order of approximately several tens of micrometers are generated,so that degradation in the appearance of body shape disadvantageouslyoccurs.

To solve the above problem, a steel sheet (BH steel sheet) which has lowstrength in press forming to be easily pressed and which exhibits highbake hardenability in a paint baking step performed after pressformation has been developed. This steel sheet is obtained bycontrolling a dissolved C amount by addition of Ti and Nb toultralow-carbon steel used as a base material, has superior surfacedistortion resistance since a yield stress (hereinafter referred to as“YP” in some cases) is low, such as approximately 240 MPa at a strengthlevel of 340 MPa, and ensures dent resistance by increasing a yieldstress (YP′) after press forming and paint baking to approximately 300MPa.

However, in view of the weight reduction, a steel sheet having athickness smaller than that of a current 340BH steel sheet having athickness of 0.65 to 0.80 mm has been desired and, for example, toreduce the thickness by 0.05 mm, the yield stress (YP′) after pressformation and paint baking must be increased to approximately 350 MPa ormore. In addition, to ensure a high YP′ while a low YP is maintained, asteel sheet is required which has high paint bake hardenability(hereinafter referred to as “BH” in some cases) and work hardenability(hereinafter referred to as “WH” in some cases).

From the situation as described above, for example, in JapaneseUnexamined Patent Application Publication No. 6-122940, a method forobtaining a steel sheet which has a low yield stress besides high WH andBH properties and which further has superior anti-aging property hasbeen disclosed in which annealing and cooling conditions of steelcontaining 0.005% to 0.0070% of C, 0.01% to 4.0% of Mn, and 0.01% to3.0% of Cr are appropriately controlled, and in which the microstructureobtained by annealing is made to be a single phase transformed in lowtemperature.

In addition, in Japanese Unexamined Patent Application Publication No.2005-281867, a method for obtaining a steel sheet having superioranti-aging property, shape fixability, and dent resistance besides highWH and BH properties and a low yield stress of 300 MPa or less has beendisclosed. In that method, the steel sheet is formed by appropriatelycontrolling annealing and cooling conditions of steel containing 0.04%or less of C, 0.5% to 3.0% of Mn, and 0.01% to 1.0% of Mo so that afterannealing, a composite microstructure is obtained which includes 0.5% toless than 10% of retained austenite on a volume fraction basis and thebalance being ferrite and a hard phase composed of bainite and/ormartensite.

In Japanese Unexamined Patent Application Publication No. 2006-233249, amethod for obtaining a steel sheet having high strength and high bakehardenability (BH) has been disclosed, which is obtained byappropriately controlling cooling conditions after annealing for steelcontaining 0.01% to less than 0.040% of C, 0.3% to 1.6% of Mn, 0.5% orless of Cr, 0.5% or less of Mo, and 1.3% to 2.1% of Mn+1.29 Cr+3.29 Moso that the microstructure after annealing includes, on a volumefraction basis, 70% or more of ferrite and 1% to 15% of martensite.

In Japanese Unexamined Patent Application Publication No. 2006-52465, amethod for obtaining a steel sheet having superior bake hardenability,anti-aging property, and press-formability has been disclosed, which isobtained by the steps of, after steel containing 0.0025% to less than0.04% of C, 0.5% to 2.5% of Mn, and 0.05% to 2.0% of Cr is annealed at atemperature between the Ac1 transformation point and less than the Ac3transformation point, performing cooling at a cooling rate of 15 to 200°C./s in the temperature range of 650 to 450° C., and further performingcooling at a cooling rate of less than 10° C./s in the temperature rangedefined by the amounts of C, Mn, and Cr.

However, the above conventional techniques have the following problems.

For example, in the technique disclosed in Japanese Unexamined PatentApplication Publication No. 6-122940, as the evaluation of theanti-aging property, the restoration amount of yield point elongation(hereinafter referred to as “YPEl” in some cases) after an artificialaging treatment at 100° C. for 1 hour was used. However, when anequivalent aging time at 30° C. is calculated using Hundy's equationshown by equation (1) (disclosed by Hundy, B. B “Accelerated StrainAgeing of Mild Steel”, J. Iron & Steel Inst., 178, pp. 34 to 38,(1954)), it is approximately 18 days at 30° C., and the techniquedescribed above cannot be always said to be superior in terms ofanti-aging property. In addition, to obtain a single phasemicrostructure transformed in low temperature, for example, annealingwas performed at an extremely high temperature region of 860 to 980° C.However, in that case, troubles, such as sheet breakage, may occur insome cases. Accordingly, development is required to form a steel sheethaving superior anti-aging property without performing high temperatureannealing.

Log₁₀(t _(r) /t)=4,400(1/T _(r)−1/T)−log₁₀(T/T _(r))  (1)

-   -   T: Acceleration aging temperature (K)    -   T_(r): Temperature (K) to be evaluated    -   t: Aging time (Hr) at acceleration aging temperature T    -   t_(r): Equivalent aging hour (Hr) converted at temperature        T_(r) (K) to be evaluated

According to the technique described in Japanese Unexamined PatentApplication Publication No. 2005-281867, to enhance work hardenability(WH), 0.01% to 1.0% and preferably 0.1% to 0.6% of Mo is contained, andas a microstructure, retained austenite is used. However, since Mo is avery expensive element, and when 0.18% to 0.56% of Mo is added asdisclosed in the example, the cost is considerably increased. On theother hand, in the comparative example among the examples in which theaddition amount of Mo is extremely low, YR is high, and WH is extremelylow. Accordingly, development of a steel sheet having a low YR and ahigh WH must be performed without using expensive Mo.

According to the technique disclosed in Japanese Unexamined PatentApplication Publication No. 2006-233249, the volume fraction ofmartensite and the solute C in ferrite are controlled, and as coolingafter annealing to obtain high bake hardenability, cooling is performedfrom a temperature of 550 to 750° C. to a temperature of 200° C. or lessat a cooling rate of 100° C./s. However, to satisfy the coolingconditions as described above, a specific method must be performed inwhich, for example, quenching is performed in jet water, as disclosed inJapanese Unexamined Patent Application Publication No. 2006-233249, andit is difficult to perform manufacturing in a current continuousgalvanizing line.

According to the technique disclosed in Japanese Unexamined PatentApplication Publication No. 2006-52465, when cooling is performed afterannealing, cooling in the temperature range of 650 to 450° C. isperformed at a cooling rate of 15 to 200° C./s, and cooling in thetemperature range defined by the amounts of C, Mn, and Cr is performedat less than 10° C./s. According to the example, cooling from theannealing temperature to 680° C. is performed at 3° C./s, rapid coolingis performed at a rate of 80° C./s to a temperature represented by Ts,slow cooling is performed at a rate of less than 10° C./s to atemperature represented by Tf, and subsequently, cooling to 180° C. andthen to room temperature are performed at 15° C./s and 100° C./s,respectively. The technique described above can be performed in a CALwhich performs no galvanizing treatment and which is provided with anoveraging zone. However, the technique is difficult to perform in a CGLwhich performs a galvanizing treatment during cooling and which is notgenerally provided with an overaging apparatus (when a galvanizingtreatment is performed, a steel sheet must be dipped in a galvanizingbath at a temperature of approximately 460° C. for several seconds, andwhen alloying is further performed, a steel sheet must be heated to 500to 600° C. and maintained for several tens of seconds). In addition,when a CGL, which has a galvanizing treatment apparatus, is providedwith an overaging zone, the line length is extremely increased. Hence,in general, an overaging zone is not provided therefor, and after agalvanizing treatment, gas cooling is performed. Hence, the case shownin the example in which the temperature range of 650 to 450° C. iscooled at a rate of 15° C./s or more, and the temperature range of 390°C. or less is cooled at an extremely low cooling rate of approximately1.3° C./s is difficult to perform by a heat cycle performed in a currentCGL. After cooling is performed to room temperature in accordance withthe above cooling pattern, galvanizing may be performed. However, thecost is seriously increased in this case. Hence, it is necessary todevelop a method for obtaining a superior material by a general heattreatment cycle in a CGL without using the thermal history as describedabove.

It could therefore be helpful to provide a high strength galvanizedsteel sheet having a low yield stress and superior anti-aging propertyand bake hardenability and a method for manufacturing the high strengthgalvanized steel sheet and, even if a general heat treatment cycle in aCGL is performed, this high strength galvanized steel sheet can beobtained without using expensive Mo.

SUMMARY

We found that, by controlling Mn and Cr, which have high hardeningproperties in a specific region, pearlite and bainite are suppressedeven in a heat treatment cycle in a CGL, which has a low cooling rate.Hence, a low yield stress and high work hardenability can be obtained.

In addition, at the same Mn equivalent, as the Mn content is decreased,the A1 and A3 lines in a Fe—C phase diagram are shifted to a highertemperature side and a higher C content side. Hence the amount of soluteC in ferrite is increased. Accordingly, when the Mn content isdecreased, the BH property, which is a strain aging phenomenon of soluteC, is improved. However, when the Mn content is excessively decreased,the aging property is degraded. Hence, it is important to control the Mncontent in an appropriate range to simultaneously obtain the anti-agingproperty and the bake hardenability.

That is, we found that when the anti-aging property and the bakehardenability are well balanced at a high level by an appropriatecontrol of the Mn content, and when the Mn equivalent (=Mn+1.3Cr) iscontrolled in an appropriate range by adjustment of the Cr content, ahigh strength galvanized steel sheet having a low yield stress and ahigh work hardenability can be manufactured.

We thus provide:

-   -   [1] A high strength galvanized steel sheet has a composition        containing, on a mass percent basis, 0.01% to less than 0.08% of        C, 0.2% or less of Si, more than 1.0% to 1.8% of Mn, 0.10% or        less of P, 0.03% or less of S, 0.1% or less of Al, 0.008% or        less of N, more than 0.5% of Cr, and the balance being iron and        inevitable impurities, in which 1.95≦Mn (mass percent)+1.3Cr        (mass percent)≦2.8 holds, wherein the microstructure includes a        ferrite phase and 2% to 15% of martensite on an area ratio        basis, and the total area ratio of pearlite and/or bainite is        1.0% or less.    -   [2] According to the above [1], in the high strength galvanized        steel sheet having excellent press formability, on a mass        percent basis, the Cr content is more than 0.65%, and the Mn        content is more than 1.0% to 1.6%.    -   [3] According to the above [1] or [2], in the high strength        galvanized steel sheet having excellent press formability, the        composition further contains, on a mass percent basis, 0.01% or        less of B.    -   [4] According to one of the above [1] to [3], in the high        strength galvanized steel sheet having excellent press        formability, the composition further contains, on a mass percent        basis, at least one selected from 0.15% or less of Mo, 0.5% or        less of V, 0.1% or less of Ti, and 0.1% or less of Nb.    -   [5] A method for manufacturing a high strength galvanized steel        sheet, comprises the steps of: performing hot rolling and cold        rolling of a steel slab having the composition according to one        of the above [1] to [4]; then performing annealing at an        annealing temperature of more than 750° C. to less than 820° C.;        performing cooling at an average cooling rate of 3 to 15° C./s        in a temperature range from the annealing temperature to a        temperature at which dipping into a galvanizing bath is        performed; performing galvanizing; and then performing cooling        at an average cooling rate of 5° C./s or more.    -   [6] According to the above [5], the method for manufacturing a        high strength galvanized steel sheet having excellent press        formability further comprises the step of, after the step of        performing galvanizing, performing an alloying treatment of a        galvanizing layer.

DETAILED DESCRIPTION

% indicating the composition of steel is always on a mass percent basis.In addition, the high strength galvanized steel sheet is a galvanizedsteel sheet having a tensile strength of 340 MPa or more.

A high strength galvanized steel sheet having a low yield stress andsuperior anti-aging property and bake hardenability can be obtained. Asa result, when the above steel sheet is used for automobile inner andouter panel application, weight reduction can also be achieved bythickness reduction.

In addition, since the high strength galvanized steel sheet has thesuperior properties described above, besides an automobile steel sheet,it can be widely used for home electric appliance application and thelike. Hence, the steel sheet has industrial advantages.

The composition is defined such that the Mn content is more than 1.0% to1.8% and the Cr content is more than 0.5, and in addition, the Mnequivalent is controlled in an appropriate range that satisfies 1.9≦Mn(mass percent)+1.3Cr (mass percent)≦2.8. In addition, the microstructureis designed such that a ferrite phase and 2% to 15% of martensite on anarea ratio basis are included, and that the total area ratio of pearliteand/or bainite is 1.0% or less. These are the most important features.When the composition and the microstructure as described above areprepared, as a result, a high strength galvanized steel sheet having alow yield stress and superior anti-aging property and bake hardenabilitycan be obtained.

In addition, to manufacture the high strength galvanized steel sheet asdescribed above which has a low yield stress and superior anti-agingproperty and bake hardenability, annealing/galvanizing conditions mustbe controlled, and annealing is performed at an annealing temperature ofmore than 750° C. to less than 820° C., cooling is performed at anaverage cooling rate of 3 to 15° C./s in a temperature range from theannealing temperature to a temperature at which dipping into agalvanizing bath is performed, and after galvanizing is performed,cooling is performed at an average cooling rate of 5° C./s or more.

Hereinafter, our steels and methods will be described in detail.

First, the reasons for selecting chemical compositions of the steel willbe described.

C: 0.01% to Less than 0.08%

C is effective to increase strength and is one of important elements.The content is set to 0.01% or more to ensure a predetermined amount ormore of martensite. On the other hand, when the C content is 0.08% ormore, since the amount of martensite is excessively large, YP isincreased, the BH amount is decreased, and in addition, the weldabilityis degraded. Hence, the C content is set to less than 0.08% and, toobtain a lower YP and a higher BH, the C content is preferably set toless than 0.06% and more preferably set to 0.05% or less.

Si: 0.2% or Less

Si has a high solid-solution strengthening ability, and a lower Sicontent is preferable in terms of decrease in yield strength (decreasein YP). However, since a Si content of up to 0.2% is permissible, the Sicontent is set to 0.2% or less.

Mn: More than 1.0% to 1.8%

Mn is the most important element. When the Mn content is more than 1.8%,the amount of solute C in ferrite is decreased, and the BH property isdegraded. In addition, when the Mn content is 1.0% or less, a high BHproperty is obtained since the amount of solute C in ferrite is large.However, on the other hand, the anti-aging property may be degraded insome cases. Hence, to simultaneously obtain the BH property and theanti-aging property, the Mn content is set in the range of more than1.0% to 1.8% and is preferably set in the range of more than 1.0% to1.6%.

P: 0.10% or Less

P is an effective element to increase strength. However, when the Pcontent is more than 0.10%, the yield strength (YP) is increased, andsurface-distortion resistance is degraded. Furthermore, an alloyingspeed of a galvanizing layer is decreased, surface defect occur, and inaddition, resistance against secondary work-embrittlement is degradeddue to segregation in grain boundaries of a steel sheet. Accordingly,the P content is set to 0.10% or less.

S: 0.03% or Less

S degrades ductility in hot rolling and enhances sensitivity of crackingin hot rolling. Hence, the content is preferably decreased. Further,when the S content is more than 0.03%, the ductility of the steel sheetis degraded due to precipitation of fine MnS, and the press formabilityis degraded. Hence, the S content is set to 0.03% or less. In addition,in view of the press formability, the S content is preferably set to0.015% or less.

Al: 0.1% or Less

Al decreases inclusions in steel as a deoxidizing element and, inaddition, it also functions to fix unnecessary solute N in steel in theform of a nitride. However, when the Al content is more than 0.1%,alumina inclusions in the form of clusters are increased, the ductilityis degraded, and the press formability is also degraded. Hence, the Alcontent is set to 0.1% or less. To effectively use Al as a deoxidizingelement and to sufficiently decrease oxygen in steel, 0.02% or more ofAl is preferably contained.

N: 0.008% or Less

Since N in a solid solution state is not preferably present in view ofanti-aging property, the content is preferably decreased. In particular,when the N content is more than 0.008%, the amount of a nitride formingelement necessary to fix N is increased. Hence, manufacturing cost isincreased. In addition, due to excessive generation of nitrides, theductility and toughness are degraded. Hence, the N content is set to0.008% or less. The N content is preferably set to less than 0.005% toensure ductility and toughness.

Cr: More than 0.5%

Cr is a hardenability improving element and is a very important elementfor formation of martensite. In addition, since having a highhardenability and a low solid-solution hardenability as compared tothose of Mn, Cr is effective to decrease YP, and Cr is positively added.However, when the Cr content is 0.5% or less, the hardenability and YPdecreasing effect may not be obtained in some cases. Hence, the contentis set to more than 0.5% and is preferably more than 0.65%.

In addition, as described above, to simultaneously obtain the BHproperty and the anti-aging property, the content of Mn is limited.Hence, even in a heat treatment cycle in a CGL in which the cooling rateis low, to suppress pearlite and bainite and to decrease YP, the Mnequivalent must be controlled to be a predetermined level by adjustingthe Cr content. Accordingly, the Cr content is set to more than 0.5% andis preferably set to more than 0.65%.

Mn+1.3Cr: 1.9% to 2.8%

The value of Mn+1.3Cr is one index indicating the hardenability and itis important to control the value to form martensite. When the value ofMn+1.3Cr is less than 1.9%, the hardenability becomes insufficient, andpearlite and bainite are liable to be generated during cooling performedafter annealing, so that YP is increased. On the other hand, when thevalue of Mn+1.3Cr is more than 2.8, the hardenability effect issaturated, and by excessive addition of alloying elements, manufacturingcost is increased. Hence, the value of Mn+1.3Cr is set in the range of1.9% to 2.8% and is preferably set in the range of more than 2.3% to2.8%.

Targeted properties of the steel can be obtained by those essentialaddition elements described above. However, besides those elements,whenever necessary, the following elements may also be added.

B: 0.01% or Less

B is a hardenability improving element and can be added in an amount of0.0005% or more to stably form martensite. Furthermore, when 0.0015% to0.004% of B is added, besides improvement in grain growth properties offerrite, BH can be improved, and balance between decrease in YP andincrease in BH can be further improved. However, when more than 0.01% ofB is added, adverse influence on the mechanical properties and theproductivity in casting are enhanced. Hence, when B is added, thecontent thereof is set to 0.01% or less. At least one of Mo: 0.15% orless, V: 0.5% or less, Ti: 0.1% or less, and Nb: 0.1% or less

Mo: 0.15% or Less

Mo is an expensive element and is an element to increase YP. However, Mois also an effective element which improves zinc coating surfacequality, or improves hardenability and stably obtains martensite, and0.01% or more of Mo may be added. However, when the Mo content is morethan 0.15%, the effects thereof is saturated, and cost is seriouslyincreased. Hence, when Mo is added, 0.15% or less of Mo may be added sothat an adverse influence thereof, increase in YP, is not sosignificant. In view of reduction in cost and decrease in YP, thecontent of Mo is preferably decreased as small as possible, and Mo ispreferably not to be added (0.02% or less of Mo being present as aninevitable impurity).

V: 0.5% or Less

V is a hardenability improving element and may be added in an amount of0.01% or more to stably form martensite. However, even when V isexcessively added, an effect corresponding to the cost cannot beobtained. Hence, when V is added, the content thereof is set to 0.5% orless.

Ti: 0.1% or Less, and Nb: 0.1% or Less

Ti and Nb each form carbide, nitride and carbonitride and decrease theamounts of solute C and N, and to prevent degradation of mechanicalproperties during aging, each element in an amount of 0.01% or more maybe added. However, even when the element in an amount of more than 0.1%is excessively added, the effect is saturated, and an effectcorresponding to the cost cannot be obtained. Hence, when Ti and/or Nbis added, the content of each element is set to 0.1% or less.

In addition, the balance other than those elements described aboveincludes Fe and inevitable impurities. As the inevitable impurities, forexample, since O forms non-metal inclusions and has an adverse influenceon the quality, the content of O is preferably decreased to 0.003% orless.

Next, the microstructure of the high strength galvanized steel sheethaving excellent press formability will be described.

A Ferrite Phase and 2% to 15% of Martensite on an Area Ratio Basis

The galvanized steel sheet has a dual phase microstructure containing aferrite phase and 2% to 15% of martensite on an area ratio basis. Whenthe martensite is controlled in the range described above, thesurface-distortion resistance and work-hardenability are improved, sothat a steel sheet usable for automobile outer panel application can beobtained. When the area ratio of the martensite is more than 15%, thestrength is significantly increased, and for example, as a steel sheetfor an automobile inner/outer plate panel, that is typically intended,sufficient surface-distortion resistance and press formability cannot beobtained. Hence, the area ratio of martensite is set to 15% or less. Onthe other hand, when the area ratio of martensite is less than 2%, YPElis liable to remain and, in addition, YP is increased, so that thesurface-distortion resistance is degraded. Hence, the area ratio ofmartensite is set in the range of 2% to 15% and is preferably set in therange of 2% to 10%.

Total Area Ratio of Pearlite and Bainite: 1.0% or Less

In the case in which slow cooling is performed after annealing and, inparticular, an alloying treatment is performed, when the Mn equivalentis not optimized, fine pearlite or bainite is generated primarilyadjacent to martensite, so that YR is increased. That is, since YP canbe decreased when the total area ratio of pearlite and/or bainite is setto 1.0% or less, this total area ratio is set to 1.0% or less.

In addition, besides the ferrite phase, martensite, pearlite, andbainite, retained γ and/or inevitable carbides having an area ratio ofapproximately 1.0% may be contained.

Here, the area ratio can be obtained by the steps of polishing an Lcross-section (vertical cross-section parallel to a rolling direction)of a steel sheet, etching the cross-section using nital, observing 12visual fields at a magnification of 4,000 times power using a SEM, andperforming image analysis of an obtained microstructure photograph. Inthe microstructure photograph, a blackish contrast region indicatesferrite, a region in which carbides are generated in the form oflamellas or points is regarded as pearlite and bainite, and particleshaving a white contrast are regarded as martensite.

When the Mn equivalent and the cooling conditions after annealing areappropriately controlled, the microstructure can be controlled in theabove area ratio range.

Next, conditions for manufacturing the high strength galvanized steelsheet will be described. The high strength galvanized steel sheet ismanufactured by the steps of forming a slab by melting steel adjusted inthe above chemical composition range; then performing hot rolling,followed by (pickling) cold rolling; then, after annealing, performingcooling at an average cooling rate of 3 to 15° C./s in a temperaturerange from the annealing temperature to a temperature at which dippinginto a galvanizing bath is performed; and after galvanizing, performingcooling at an average cooling rate of 5° C./s or more.

In this case, the method for melting and refining steel is notparticularly limited, and an electric furnace may be used, or aconverter may be used. In addition, as a method for casting steel afterthe melting and refining, a cast slab may be formed by a continuouscasting method, or an ingot may be formed by an ingot-making method.

When the slab is hot-rolled after continuous casting, rolling may beperformed after the slab is re-heated in a heating furnace, or directrolling may be performed without heating the slab. In addition, afterblooming is performed for the ingot thus formed, hot rolling may beperformed.

Hot rolling may be performed in accordance with an ordinary method, forexample, such that the temperature for heating the slab is set to 1,100to 1,300° C., the finish rolling temperature is set to the Ar3 point ormore, the cooling rate after the finish rolling is set to 10 to 200°C./s, and the coiling temperature is set to 400 to 750° C. The reductionratio of cold rolling may be set to 50 to 85% which is the rangeperformed in a general operation.

Hereinafter, annealing and galvanizing steps (CGL process) will bedescribed in detail.

Annealing Temperature: More than 750° C. to Less than 820° C.

The annealing temperature must be increased to an appropriatetemperature to obtain a microstructure containing a ferrite phase andmartensite. When the annealing temperature is 750° C. or less, sinceaustenite is not sufficiently formed, a predetermined amount ofmartensite cannot be obtained. Hence, for example, due to remainingYPEl, increase in YP, the surface-distortion resistance is degraded. Onthe other hand, when the annealing temperature is 820° C. or more, theamount of solute C in ferrite is decreased, and a high BH amount may notbe obtained in some cases. In addition, enrichment of C in austenite isnot sufficiently performed, and pearlite and bainite are liable to begenerated during the subsequent cooling and alloying treatments, so thatincrease in YP ccurs. Hence, the annealing temperature is set to morethan 750° C. to less than 820° C.

Primary Average Cooling Rate: 3 to 15° C./s

In manufacturing the galvanized steel sheet, after the annealing, theprimary average cooling rate from the annealing temperature to atemperature at which dipping into a galvanizing bath is performed is setto 3 to 15° C./s. When the cooling rate is less than 3° C./s, since thepearlite and bainite significantly generate during cooling, YP isincreased. In addition, since pearlite and bainite are generated, apredetermined amount of martensite cannot be obtained, and since YPElremains, YP is increased. On the other hand, when the cooling rate ismore than 15° C./s, enrichment of C, Mn, and Cr in austenite is notsufficiently performed, and austenite is decomposed into pearlite andbainite during the subsequent cooling and alloying treatments, so thatthe amounts thereof are increased. Hence, YP is increased. In addition,enrichment of C in ferrite is suppressed, and a high BH amount may notbe obtained in some cases. Hence, after the annealing, the primaryaverage cooling rate is set to 3 to 15° C./s from the annealingtemperature to a temperature at which dipping into a galvanizing bath isperformed. A preferable average cooling rate is 5 to 15° C./s. Inaddition, a galvanizing bath temperature in a galvanizing treatment maybe a common temperature, such as approximately 400 to 480° C.

In addition, after the galvanizing treatment is performed, whenevernecessary, the alloying treatment may be performed. In this case, thealloying treatment after the galvanizing is performed, for example, suchthat after the dipping in a galvanizing bath is performed, whenevernecessary, heating is performed to a temperature range of 500 to 700°C., and the temperature is maintained for several seconds to severaltens of seconds. According to a conventional steel sheet in which the Mnequivalent is not specified, the mechanical properties are seriouslydegraded by the alloying treatment as described above. However,according to our steels, the increase in YP is small even if thealloying treatment as described above is performed.

In addition, as the galvanizing conditions, a coating amount per onesurface is preferably 20 to 70 g/m², and when the alloying treatment isperformed, the Fe content in the coating layer is preferably set to 6%to 15%.

Secondary Cooling Rate: 5° C./s or More

In the secondary cooling to be performed after the galvanizing treatmentor the alloying treatment, to obtain a predetermined amount ofmartensite, cooling is performed at an average cooling rate of 5° C./sor more to a temperature of the Ms point or less. By slow cooling inwhich the secondary cooling rate is less than 5° C./s, pearlite orbainite is generated at approximately 400 to 500° C., so that YP isincreased. On the other hand, although it is not necessary to limit theupper limit of the secondary cooling rate, when it is more than 100°C./s, martensite is excessively hardened, so that the ductility isdegraded. Hence, the second cooling rate is preferably 100° C./s orless. Accordingly, the secondary cooling rate is set to 5° C./s or moreand is preferably set to 10 to 100° C./s.

Furthermore, temper rolling may also be performed on the steel sheetafter the heat treatment for shape flattening. In addition, a steelmaterial is supposed to be manufactured by the steps including generalsteel making, casting, and hot rolling. However, by omitting part of thehot rolling step or all thereof, a steel material may be manufactured,for example, by thin-slab casting.

In addition, the surface of the galvanized steel sheet may be furtherprocessed by an organic film treatment.

EXAMPLES Example 1

Hereinafter, our steel sheets and methods will be further described withreference to Examples.

Several types of steel having chemical compositions of steel A to Yshown in Table 1 were melted by vacuum melting, so that slabs wereformed. After these slabs were heated to 1,200° C. and were then hotrolled at a finish temperature of 850° C., cooling was performed, andcoiling was then performed at 600° C., so that a hot-rolled band havinga thickness of 2.5 mm was manufactured. After pickling was performed forthe hot-rolled band thus obtained, cold rolling was performed at areduction ratio of 70%, so that a cold-rolled steel sheet having athickness of 0.75 mm was obtained.

TABLE 1 Chemical Compositions (Mass Percent) Mn + 1.3Cr Steel C Si Mn PS Sol. Al N Cr Others (%) Remarks A 0.042 0.02 1.08 0.010 0.015 0.0350.0045 0.70 — 1.99 Invention Steel B 0.034 0.01 1.25 0.009 0.005 0.0400.0043 0.71 — 2.17 Invention Steel C 0.028 0.01 1.42 0.009 0.005 0.0400.0043 0.71 — 2.34 Invention Steel D 0.021 0.02 1.56 0.013 0.005 0.0500.0040 0.72 — 2.50 Invention Steel E 0.013 0.01 1.78 0.009 0.012 0.0400.0040 0.71 — 2.70 Invention Steel F 0.035 0.01 1.57 0.015 0.009 0.0400.0040 0.57 — 2.31 Invention Steel G 0.032 0.02 1.56 0.010 0.015 0.0350.0050 0.72 — 2.50 Invention Steel H 0.028 0.02 1.55 0.011 0.005 0.0350.0050 0.92 — 2.75 Invention Steel I 0.022 0.01 1.56 0.009 0.005 0.0400.0043 0.71 — 2.48 Invention Steel J 0.032 0.02 1.58 0.013 0.005 0.0500.0040 0.73 — 2.53 Invention Steel K 0.042 0.01 1.57 0.009 0.012 0.0400.0040 0.71 — 2.49 Invention Steel L 0.074 0.01 1.56 0.011 0.025 0.0400.0040 0.72 B: 0.0017 2.50 Invention Steel M 0.032 0.01 1.58 0.011 0.0090.030 0.0055 0.71 — 2.50 Invention Steel N 0.031 0.05 1.57 0.011 0.0150.035 0.0035 0.71 Mo: 0.08 2.49 Invention Steel O 0.033 0.05 1.56 0.0110.005 0.050 0.0050 0.70 Mo: 0.05 2.47 Invention B: 0.002 Steel P 0.0330.05 1.55 0.010 0.012 0.035 0.0040 0.71 Ti: 0.02 2.47 Invention V: 0.05Steel Q 0.032 0.02 1.58 0.011 0.009 0.035 0.0045 0.71 Nb: 0.01 2.50Invention Steel R 0.028 0.02 1.85 0.011 0.005 0.050 0.0040 0.45 — 2.44Comparative Steel S 0.050 0.02 0.95 0.010 0.005 0.050 0.0040 0.90 — 2.12Comparative Steel T 0.055 0.02 2.02 0.011 0.005 0.040 0.0035 0.58 — 2.77Comparative Steel U 0.050 0.02 1.12 0.009 0.005 0.050 0.0040 0.52 — 1.80Comparative Steel V 0.004 0.01 1.77 0.011 0.025 0.040 0.0040 0.77 — 2.77Comparative Steel W 0.040 0.02 1.02 0.026 0.012 0.081 0.0018 0.7 B:0.0015 1.93 Invention Steel X 0.029 0.01 1.37 0.018 0.005 0.064 0.00370.6 B: 0.0031 2.15 Invention Steel Y 0.028 0.01 1.38 0.008 0.0008 0.0560.0029 0.64 B: 0.0015, 2.21 Invention Ti: 0.004 Steel

Next, samples obtained by cutting off from the cold-rolled steel sheets,which were obtained as described above, were sequentially processed bythe steps of performing annealing at annealing temperatures shown inTable 2 for 60 seconds in an infrared image furnace; performing primarycooling under conditions shown in Table 2; performing galvanizing(galvanizing bath temperature: 460° C.); performing an alloyingtreatment (520° C.×15 s); performing secondary cooling to a temperatureof 150° C. or less; and performing temper rolling at an extension rateof 0.4%. In this case, the galvanizing treatment was adjusted to have acoating weight of 50 g/m² per one surface, and the alloying treatmentwas adjusted so that the Fe content in the coating layer was 9% to 12%.

From the galvanized steel sheets obtained as described above, sampleswere obtained, and the area ratio of martensite and the total area ratioof pearlite and/or bainite were measured. In addition, the tensileproperties, work hardening amount (WH), bake hardening amount (BH), andyield point elongation (YPEl) obtained after an acceleration aging testwere measured. The detailed measurement methods are described below:

-   -   (1) Area ratio of Martensite: After an L cross-section (vertical        cross-section parallel to the rolling direction) was        mechanically polished and was then etched with nital, 12 visual        fields were observed by a scanning electron microscope (SEM) at        a magnification of 4,000 times power, and quantification was        performed using an obtained photograph (SEM photograph) of        microstructure. In the photograph, particles having a white        contrast were regarded as martensite, and remaining parts having        a black contrast were regarded as ferrite, so that the ratio of        martensite with respect to the overall area was obtained.    -   (2) Tensile Properties: JIS No. 5 test pieces were obtained in a        90°-direction (C direction) with respect to the rolling        direction, and a tensile test in accordance with JIS Z2241 was        performed, so that the yield stress (YP) and the tensile        strength (TS) were measured.    -   (3) Work Hardenability Amount (WH): The difference between a        stress at a pre-strain of 2% and the yield stress (YP) was        measured.    -   (4) Bake Hardenability Amount (BH): The difference between a        stress at a pre-strain of 2% and the yield stress obtained by a        heat treatment corresponding to paint baking at 170° C. for 20        minutes.    -   (5) Yield Point Elongation (YPEl) after Acceleration Aging Test:        After a heat treatment at 100° C. for 24 hours, YPEl was        measured by the tensile test (in accordance with HS Z2241). In        consideration of the case in which a steel sheet crosses the red        line for export, the acceleration aging conditions were set so        that the equivalent aging times obtained from Hundy's equation        were 1.2 years at 30° C. and approximately 2 months at 50° C.

The measurement results are shown in Table 2 together with themanufacturing conditions.

TABLE 2 Annealing and Galvanizing Conditions Temper MicrostructurePrimary Secondary Rolling Total Area Annealing Cooling Cooling ExtensionPrimary Martensite Ratio of Steel Temperature Rate Alloying Rate RateMicrostructure Area Ratio Pearlite and No. No. (° C.) (° C./s)Conditions (° C./s) (%) E* (%) Bainite (%) 1 A 770 12 520° C. × 15 s 400.4 F + M + P/B 2.9 0.95 2 B 770 12 520° C. × 15 s 40 0.4 F + M + P/B2.8 0.73 3 C 780 5 520° C. × 15 s 20 0.4 F + M + P/B 2.6 0.69 4 D 780 5520° C. × 15 s 20 0.4 F + M + P/B 2.6 0.58 5 E 800 5 520° C. × 15 s 200.4 F + M + P/B 2.5 0.54 6 F 780 5 520° C. × 15 s 20 0.4 F + M + P/B 4.20.72 7 G 780 5 520° C. × 15 s 20 0.4 F + M + P/B 4.0 0.58 8 H 780 5 520°C. × 15 s 20 0.4 F + M + P/B 4.1 0.52 9 I 780 5 520° C. × 15 s 20 0.4F + M + P/B 2.9 0.58 10 J 780 5 520° C. × 15 s 20 0.4 F + M + P/B 4.60.60 11 K 780 5 520° C. × 15 s 20 0.4 F + M + P/B 5.6 0.59 12 L 780 5520° C. × 15 s 20 0.4 F + M + P/B 13.3 0.51 13 M 780 5 520° C. × 15 s 200.4 F + M + P/B 4.2 0.61 14 N 780 5 520° C. × 15 s 20 0.4 F + M + P/B4.4 0.63 15 O 780 5 520° C. × 15 s 20 0.4 F + M + P/B 4.3 0.64 16 P 7805 520° C. × 15 s 20 0.4 F + M + P/B 4.3 0.62 17 Q 780 5 520° C. × 15 s20 0.4 F + M + P/B 4.5 0.64 18 R 780 5 520° C. × 15 s 20 0.4 F + M + P/B4.5 0.53 19 S 780 5 520° C. × 15 s 20 0.4 F + M + P/B 2.2 0.96 20 T 7805 520° C. × 15 s 20 0.4 F + M + P/B 11.6 0.52 21 U 780 12 520° C. × 15 s40 0.4 F + M + P/B 1.4 1.44 22 V 780 12 520° C. × 15 s 40 0.4 F + M +P/B 1.1 0.53 40 W 785 12 520° C. × 15 s 40 0.4 F + M + P/B 4.0 0.34 41 X785 5 520° C. × 15 s 20 0.4 F + M + P/B 3.9 0.24 42 Y 785 5 520° C. × 15s 20 0.4 F + M + P/B 3.8 0.26 Mechanical Properties YPEI After YP TS YRWH BH YP′ Aging No. (MPa) (MPa) (%) (MPa) (MPa) (MPa) (%) Remarks 1 244461 52.9 59 81 384 0.2 Invention Example 2 234 455 51.4 61 78 373 0.1Invention Example 3 223 453 49.2 64 72 359 0 Invention Example 4 217 45248.0 67 66 350 0 Invention Example 5 215 451 47.7 70 55 340 0 InventionExample 6 235 477 49.3 70 66 371 0 Invention Example 7 228 473 48.2 7167 366 0 Invention Example 8 224 475 47.2 75 66 365 0 Invention Example9 219 452 48.5 65 66 350 0 Invention Example 10 237 484 49.0 72 67 376 0Invention Example 11 243 498 48.8 80 66 389 0 Invention Example 12 254529 48.0 110 61 425 0 Invention Example 13 228 475 48.0 72 66 366 0Invention Example 14 238 480 49.6 68 71 377 0 Invention Example 15 236478 49.4 67 68 371 0 Invention Example 16 238 479 49.7 71 67 376 0Invention Example 17 240 481 49.9 72 66 378 0 Invention Example 18 235482 48.8 71 48 354 0 Comparative Example 19 230 443 51.9 63 82 375 1.8Comparative Example 20 272 566 48.1 102 41 415 0 Comparative Example 21245 441 55.6 62 78 385 1.6 Comparative Example 22 261 432 60.4 63 57 3811.2 Comparative Example 40 219 469 46.7 77 87 383 0 Invention Example 41207 454 45.6 73 79 359 0 Invention Example 42 210 458 45.9 72 80 362 0Invention Example *F: Ferrite, M: Martensite, P: Pearlite, B: Bainite

In Table 2, the compositions and the manufacturing conditions of Nos. 1to 17 and 40 to 42 are within our range, and the microstructures thereofare our examples in which the area ratio of martensite is in the rangeof 2% to 15%, and the total area ratio of pearlite and/or bainite is1.0% or less. Compared to comparative examples, our examples have a lowYR and a high BH, and YPEl after aging is also low, such as 0.2% orless.

On the other hand, according to Nos. 18 to 22 of the comparativeexamples manufactured using steel R to V which are outside ourpredetermined composition, at least one of YR, BH, and YPEl after agingare inferior.

As for No. 18 (steel R), the Mn content and the Cr content are outsideour range and, since the Mn content is particularly high, the BH amountis low. As for No. 19 (steel S), since the Mn content is low, the amountof solute C in ferrite is large, and a high BH is obtained. However, onthe other hand, YPEl after aging is high, so that the anti-agingproperty is inferior. As for No. 20 (steel T), since the Mn content ishigh, the amount of solute C in ferrite is small, so that BH is low. Inaddition, since ferrite is sold-solution strengthened, YP is relativelyhigh, and the surface-distortion resistance is inferior. As for No. 21(steel U), since the value of Mn+1.3Cr is low, pearlite and bainite aregenerated during cooling performed after annealing, and a predeterminedamount of martensite can not be ensured. Hence, YR is relatively high,and YPEl after aging is also high. As for No. 22 (steel V), since theamount of C is small, a predetermined amount of martensite can not beobtained. Hence, YR is high, and YPEl after aging is also high.

Example 2

Several types of steel having chemical compositions of steel C, D, E,and G shown in Table 1 were melted by vacuum melting, and underconditions similar to those in Example 1, they were then processed byhot rolling, pickling, and cold rolling, followed by annealing atannealing temperatures shown in Table 3 for 60 seconds. Subsequently,after primary cooling under conditions shown in Table 3 and agalvanizing treatment (galvanizing bath temperature: 460° C.) wereperformed, an alloying treatment was performed, and secondary cooling toa temperature of 150° C. or less and temper rolling were then performed.

Samples were obtained from the galvanized steel sheets thus obtained,and by methods similar to those in Example 1, the area ratio ofmartensite and the total area ratio of pearlite and/or bainite weremeasured. In addition, the tensile properties, work hardenability amount(WH), bake hardenability amount (BH), and YPEl after an accelerationaging test were measured.

The obtained results are shown in Table 3 together with themanufacturing conditions.

TABLE 3 Annealing and Temper Microstructure Plating Conditions RollingTotal Area Annealing Primary Secondary Extension Martensite Ratio ofSteel Temperature Cooling Rate Alloying Cooling Rate Rate Primary AreaRatio Pearlite and No. No. (° C.) (° C./s) Conditions (° C./s) (%)Microstructure* (%) Bainite (%) 23 C 780 5 520° C. × 15 s 20 0.4 F + M +P/B 2.6 0.69 24 740 5 520° C. × 15 s 20 0.4 F + M + P/B 1.4 0.32 25 7605 520° C. × 15 s 20 0.4 F + M + P/B 2.5 0.66 26 800 5 520° C. × 15 s 200.4 F + M + P/B 3.2 0.72 27 840 5 520° C. × 15 s 20 0.4 F + M + P/B 4.31.06 28 D 780 5 520° C. × 15 s 20 0.4 F + M + P/B 2.6 0.58 29 780 5 520°C. × 15 s 20 NONE F + M + P/B 2.7 0.56 30 800 5 NONE 20 0.4 F + M + P/B3.3 0.34 31 E 800 5 520° C. × 15 s 20 0.4 F + M + P/B 2.5 0.54 32 800 2520° C. × 15 s 20 0.4 F + M + P/B 1.1 1.45 33 770 20 520° C. × 15 s 200.4 F + M + P/B 1.3 1.31 34 780 5 520° C. × 15 s 3 0.4 F + M + P/B 1.41.41 35 800 5 520° C. × 15 s 40 0.4 F + M + P/B 3.2 0.55 36 G 780 5 520°C. × 15 s 20 0.4 F + M + P/B 4.0 0.58 37 760 5 520° C. × 15 s 20 NONEF + M + P/B 4.2 0.57 38 800 10 520° C. × 15 s 20 0.4 F + M + P/B 5.40.62 39 800 5 NONE 30 0.4 F + M + P/B 4.8 0.28 Mechanical PropertiesYPEI After YP TS YR WH BH YP′ Aging No. (MPa) (MPa) (%) (MPa) (MPa)(MPa) (%) Remarks 23 223 453 49.2 64 72 359 0 Invention Example 24 248428 57.9 52 83 383 1.0 Comparative Example 25 215 443 48.5 61 75 351 0Invention Example 26 223 459 48.6 67 68 358 0 Invention Example 27 249478 52.1 68 59 376 0 Comparative Example 28 217 452 48.0 67 66 350 0Invention Example 29 197 448 44.0 83 68 348 0 Invention Example 30 217461 47.1 69 66 352 0 Invention Example 31 215 451 47.7 70 55 340 0Invention Example 32 232 411 56.4 44 86 362 2.0 Comparative Example 33233 421 55.3 48 82 363 1.0 Comparative Example 34 236 429 55.0 49 80 3650.8 Comparative Example 35 227 459 49.5 71 53 351 0 Invention Example 36228 473 48.2 71 65 364 0 Invention Example 37 208 472 44.1 93 66 367 0Invention Example 38 243 495 49.1 79 65 387 0 Invention Example 39 240486 49.4 75 67 382 0 Invention Example *F: Ferrite, M: Martensite, P:Pearlite, B: Bainite

As shown in Table 3, the compositions and the manufacturing conditionsof Nos. 23, 25, 26, 28 to 31, and 35 to 39 are within our range, and themicrostructures thereof are our examples in which the area ratio ofmartensite is in the range of 2% to 15%, and the total area ratio ofpearlite and/or bainite is 1.0% or less. Compared to comparativeexamples, our examples have a lower YR and a higher BH, and YPEl afteraging is also smaller, such as 0.2% or less.

On the other hand, as for No. 24, since the annealing temperature islow, a predetermined amount of martensite can not be obtained, YR ishigh, and YPEl after aging is also high, so that the anti-aging propertyis inferior.

As for No. 27, since the annealing temperature is high, enrichment ofelements in austenite during annealing is insufficient. Hence, pearliteand bainite are generated during the alloying treatment. As a result,compared to our example having the same strength as that of No. 27, YRis relatively high.

As for No. 32, since the primary cooling rate is low, its cooling curvecome across pearlite and bainite noses, and the generation amountsthereof are increased, so that YP is increased. In addition, sincepearlite and bainite are generated, a predetermined amount of martensitecan not be obtained, and due to remaining YPEl, YP is increased. Hence,YR is relatively high, and YPEl after aging is also relatively high.

As for No. 33, since the primary cooling rate is high, enrichment ofelements in austenite is insufficient, and pearlite and bainite areliable to be generated during the alloying treatment. As a result, themartensite area ratio obtained after cooling is decreased, YR isrelatively high, and YPEl after aging is also high.

As for No. 34, since the secondary cooling rate is low, austenite isdecomposed into pearlite and bainite in a temperature range ofapproximately 400 to 500° C. during the secondary cooling, and theamounts thereof are increased. Hence, the martensite area ratio obtainedafter cooling is decreased. Accordingly, YR is relatively high, and YPElafter aging is also high.

INDUSTRIAL APPLICABILITY

Since our high strength galvanized steel sheet has a low yield stressand also has superior anti-aging property and bake hardenability, thesteel sheet can be applied to parts which require high formability, suchas automobile inner and outer plate application.

1. A high strength galvanized steel sheet having a composition whichcontains, on a mass percent basis, 0.01% to less than 0.08% of C, 0.2%or less of Si, more than 1.0% to 1.8% of Mn, 0.10% or less of P, 0.03%or less of S, 0.1% or less of Al, 0.008% or less of N, more than 0.5% ofCr, and the balance being iron and inevitable impurities, wherein1.95≦Mn (mass percent)+1.3Cr (mass percent)≦2.8 and the microstructureincludes a ferrite phase and 2% to 15% of martensite on an area ratiobasis, and the total area ratio of perlite and/or bainite is 1.0% orless.
 2. The high strength galvanized steel sheet according to claim 1,wherein, on a mass percent basis, the Cr content is more than 0.65%, andthe Mn content is more than 1.0% to 1.6%.
 3. The high strengthgalvanized steel sheet according to claim 1, wherein the compositionfurther contains, on a mass percent basis, 0.01% or less of B.
 4. Thehigh strength galvanized steel sheet according to claim 1, wherein thecomposition further comprises, on a mass percent basis, at least oneselected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less ofTi, and 0.1% or less of Nb.
 5. A method for manufacturing a highstrength galvanized steel sheet, comprising: performing hot rolling andcold rolling of a steel slab having the composition according to claim1; performing annealing at an annealing temperature of more than 750° C.to less than 820° C.; performing cooling at an average cooling rate of 3to 15° C./s in a temperature range from the annealing temperature to atemperature at which dipping into a galvanizing bath is performed;performing galvanizing; and performing cooling at an average coolingrate of 5° C./s or more.
 6. The method according to claim 5, furthercomprising, after performing galvanizing, performing an alloyingtreatment of a galvanizing layer.
 7. The high strength galvanized steelsheet according to claim 2, wherein the composition further contains, ona mass percent basis, 0.01% or less of B.
 8. The high strengthgalvanized steel sheet according to claim 2, wherein the compositionfurther comprises, on a mass percent basis, at least one selected from0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% orless of Nb.
 9. The high strength galvanized steel sheet according toclaim 3, wherein the composition further comprises, on a mass percentbasis, at least one selected from 0.15% or less of Mo, 0.5% or less ofV, 0.1% or less of Ti, and 0.1% or less of Nb.
 10. The high strengthgalvanized steel sheet according to claim 7, wherein the compositionfurther comprises, on a mass percent basis, at least one selected from0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% orless of Nb.
 11. A method for manufacturing a high strength galvanizedsteel sheet, comprising: performing hot rolling and cold rolling of asteel slab having the composition according to claim 2; performingannealing at an annealing temperature of more than 750° C. to less than820° C.; performing cooling at an average cooling rate of 3 to 15° C./sin a temperature range from the annealing temperature to a temperatureat which dipping into a galvanizing bath is performed; performinggalvanizing; and performing cooling at an average cooling rate of 5°C./s or more.
 12. A method for manufacturing a high strength galvanizedsteel sheet, comprising: performing hot rolling and cold rolling of asteel slab having the composition according to claim 3; performingannealing at an annealing temperature of more than 750° C. to less than820° C.; performing cooling at an average cooling rate of 3 to 15° C./sin a temperature range from the annealing temperature to a temperatureat which dipping into a galvanizing bath is performed; performinggalvanizing; and performing cooling at an average cooling rate of 5°C./s or more.
 13. A method for manufacturing a high strength galvanizedsteel sheet, comprising: performing hot rolling and cold rolling of asteel slab having the composition according to claim 4; performingannealing at an annealing temperature of more than 750° C. to less than820° C.; performing cooling at an average cooling rate of 3 to 15° C./sin a temperature range from the annealing temperature to a temperatureat which dipping into a galvanizing bath is performed; performinggalvanizing; and performing cooling at an average cooling rate of 5°C./s or more.
 14. A method for manufacturing a high strength galvanizedsteel sheet, comprising: performing hot rolling and cold rolling of asteel slab having the composition according to claim 7; performingannealing at an annealing temperature of more than 750° C. to less than820° C.; performing cooling at an average cooling rate of 3 to 15° C./sin a temperature range from the annealing temperature to a temperatureat which dipping into a galvanizing bath is performed; performinggalvanizing; and performing cooling at an average cooling rate of 5°C./s or more.
 15. A method for manufacturing a high strength galvanizedsteel sheet, comprising: performing hot rolling and cold rolling of asteel slab having the composition according to claim 10; performingannealing at an annealing temperature of more than 750° C. to less than820° C.; performing cooling at an average cooling rate of 3 to 15° C./sin a temperature range from the annealing temperature to a temperatureat which dipping into a galvanizing bath is performed; performinggalvanizing; and performing cooling at an average cooling rate of 5°C./s or more.